Another idea could be to make a $100 coin for circulation, out of 90% gold (a pretty small coin, easy to get lost

). If someone loses it it won't decompose underground like paper money and will (I hope) be worth more than face value if gold goes up in the future. Maybe not the brightest idea for the government (the price of gold changes a lot), but certainly a great thing for metal detectorists and you will maybe having the chance of getting them from bank tellers ten years from now for face value, which will be worth it if gold goes up in value.